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A small cylindrical germanium crystal doped with donor and acceptor impurities of different segregation coefficients is partly melted, then caused to freeze rapidly. Due to the impurity segregation effect a p-n junction is formed at the stopping level of the liquid-solid interface. As the freezing process is accelerated, quenching occurs and impurity segregation can no longer take place. This produces a second p-n junction very close to the first junction. It is shown that most of the heat is dissipated by conduction through the crystal. Neglecting convection and radiation losses, the freezing rate in a typical structure can be greater than 0.85 cm/ sec. It is found that the whole structure remains a single crystal after such a treatment.